Electromagnetic waves can cause malfunctions in some electronic devices. More importantly negative influence on the health of human beings is also suspected. Therefore, most of countries regulate an electromagnetic interference and resistance according to an international standard, i.e., an International Special Committee on Radio Interference (CISPR) of an International Electrotechnical Commission (IEC) and an Electromagnetic Compatibility (EMC). Therefore, in order to sell various electronic devices, the electronic devices should meet the regulation for the electromagnetic waves.
To protect the electronic devices and rooms interiors from the electromagnetic waves, an electromagnetic shielding material is mainly used. The electromagnetic material for the aforementioned purpose should possess good electric conductivity to minimize the penetration of the electromagnetic waves into the material and low magnetic permeability to convert magnetic energy into heat.
The prior art electromagnetic shielding material is typically manufactured by dispersing a metal powder or a carbon nanotube with high electrical conductivity into a polymer such as silicon rubber, polyurethane, polycarbonate and epoxy resin, wherein a volume percent of the metal powder or the carbon nanotube is higher than 30%. Herein, the metal powder uses mainly a silver powder or a silver-coated copper powder which has high electrical conductivity. When the silver powder over 30 volume % is dispersed into the polymer, it is possible to obtain the electromagnetic shielding material having a volume resistivity less than 0.01 ohm-cm and an electromagnetic shielding effectiveness of about 50 dB.
In recent years, however, there is still required for the electromagnetic shielding material having much more enhanced volume resistivity and electromagnetic shielding effectiveness, to meet the rigid regulation of the electromagnetic shielding interference. To solve the above problem, lots of metal powders such as silver powders are dispersed into the polymer. However, as metal powders are dispersed more and more into the polymer, mechanical property such as impact strength becomes deteriorated while the electromagnetic shielding effectiveness becomes enhanced. Thus, it is difficult to manufacture the electromagnetic shielding material stably when large amount of the metal powder is dispersed in the polymer.
Meanwhile, since the carbon nanotube has been developed by S. Iijima, disclosed in Nature Vol. 354, page 56, published in 1991, various researches for the carbon nanotube has been advanced up to date. The carbon nanotube has advantageous merits as followings in comparison with the other conventional materials; it has high elastic coefficient ranging from about 1.0 MPa to about 1.8 MPa; it has an enhanced heat resistant property to endure at 2,800° C. in vacuum state; its heat conductivity is two times to that of a diamond; and its current transferring capability is about 1,000 times to that of copper. Therefore, the carbon nanotube is widely applied to a nano-scaled electric/electronic device, a nano-sensor, a photoelectric device and a high-functional composite. In case of applying the carbon nanotube to the electromagnetic shielding, it is possible to obtain the electromagnetic shielding material with low volume resistivity like a semiconductor in case of dispersing the carbon nanotube beyond 0.04 volume % into the polymer, because small amount of the carbon nanotube renders a conductive network formed in the polymer.
However, the prior art electromagnetic shielding material has a shortcoming that it is difficult to obtain a desired electromagnetic shielding effectiveness. That is, though there are much more carbon nanotube dispersed into the polymer, the electromagnetic shielding material shows relatively high volume resistivity, i.e., about 10 ohm-cm and poor shielding effectiveness. Moreover, it is hard to disperse even small amount of the carbon nanotube into the polymer so that there is a limitation to apply it to the electromagnetic shielding material.